It is known that a magnetic resonance tomography machine (MRI machine) comprises, amongst others, those three functional modules which are illustrated in FIG. 2: a basic field system 11, a gradient system 12, and a radiofrequency system 13 (also called a RF-system or body resonator). The basic field system 11 is generally a magnet and provides a strong, static magnetic field. The gradient system 12 provides an adjustable magnetic field in the low frequency region up to approximately 1 kHz, which has a linearly increasing or decreasing course in one or a number of directions. In the radiofrequency region, the RF-system 13 provides, in the vicinity of the nuclear magnetic resonance frequency essentially given by the static magnetic field (in general, 42.45 MHz), an oscillating magnetic field for the excursion of the nuclear spins, which can furthermore also serve to receive the signals of the relaxing nuclear spins.
These three modules are arranged in conventional magnetic resonance tomography machines around the patient to be examined in the following order, arranged in a radial direction from the inside outwards: RF-system 13, gradient system 12 and basic field system 11. The patient lies on a couch 14, which is located radially within the RF-system 13.
Alongside magnetic resonance tomography (MRI) positron emission tomography (PET) has also become increasingly widespread of recent years in medical diagnosis. While MRI is an imaging method for displaying structures and slice images in the interior of the body, PET enables the visualization and quantification of metabolic activities in vivo.
PET uses the particular properties of positron emitters and positron annihilation in order to determine the function of organs or cell areas quantitatively. In this case, before the examination the patient is administered appropriate radiopharmaceuticals that are marked with radionuclides. In the event of decay, the radionuclides emit positrons that interact with an electron after a short distance, resulting in a so-called annihilation. Two gamma quanta are produced in this case and fly apart from one another in opposite directions (offset by 180°). The gamma quanta are detected by two opposite PET detector modules inside a specific time window (coincidence measurement), as a result of which the location of annihilation is determined at a position on the connecting line between these two detector modules.
For detection, the PET-detector modules are arranged around the patient in an annular fashion and generally cover a major part of the length of the gantry arc. When detecting a gamma quantum, each PET-detector module generates an event record that specifies the time and the detection location, that is to say the appropriate detector element. These items of information are transferred to a fast logic unit and compared. If two events coincide within a maximum time spacing, it is assumed there is a gamma decay process on the connecting line between the two associated PET-detector modules. The reconstruction of the PET image is performed with aid of a tomography algorithm, that is to say the so-called back projection.
A superimposed imaging of the two methods is desirable in many instances on the basis of the different items of information that are obtained by MRI and PET.
To combine the imaging MRI and PET methods in one machine, it is necessary to arrange inside the basic field system and the gradient system the two units of the RF system and the PET detectors required for the data acquisition. A concentric arrangement, in which the RF system would be positioned inside the annularly arranged PET detectors, would be associated with a number of difficulties.
First, the structure of the interior coil arrangement (transmitting and receiving coils) of the RF system reduces the sensitivity of the annularly arranged PET detectors, which requires a correction during the PET image reconstruction.
Also, nesting the RF system and the annularly arranged PET detectors from the inside to the outside greatly reduces the interior diameter remaining for the patient.
Moreover, the distance between the annularly arranged PET detectors and the RF conductor structures that is required for a high quality of the RF body resonator has to be greatly reduced (field return space).
Finally, due to the radial spatial conditions, it is not possible to screen the annularly arranged PET detectors (e.g. by septa) against gamma radiation from the outside of the annularly arranged PET detectors.